Th binding of heavy meromyosin (HMM), a soluble two-headed fragment of myosin, to F-actin was examined under a variety of conditions to determine the interaction occurring between the HMM heads in binding to F-actin. We find that the HMM molecules bind independently along the F-actin filament with a stoichiometry of one HMM/2 F-actin monomers in the presence of AMP-PNP or ADP, the same stoichiometry observed in the absence of nucleotide. In the absence of nucleotide (micron equals 0.22M, 22 degrees,) HMM binds to F-actin about 600-fold stronger than the single-headed myosin fragment, subfragment-one (S-1), whereas there is only a 2-fold difference between the binding of HMM and S-1 to actin in the presence of saturating concentrations of AMP-PMP. These data, as well as results obtained at varied ionic strength and temperature show that there is a fixed relationship between the binding of S-1 to actin and the binding of each of the HMM heads to actin, i.e. the binding of each of the HMM heads to actin is always affected to the same extent as the binding of S-1 to actin. This shows there is a constant ratio between the association constants for the binding of the first and second heads of HMM to F-actin, suggesting that the amount of distortion which occurs in the HMM molecule when the second HMM head binds to F-actin remains the same under all experimental conditions.